Volume Control

Using the DDJ-1000 as a standalone mixer, how can I control the volume?

When I had a DJM, I had my PC sounds (iTunes, alerts, etc.) routed through CH4 and was able to use the volume sliders (and FX) but I can't see how I can do this with this controller.

There are no options in MIDI app (Mac).

At the minute I'm having to turn the Master knob virtually all the way anti-clockwise as the volume is so high.
I've changed the settings to -12db also for both channel faders and master.

When using the controller and rekordbox as normal, I can turn the master knob to the 3 o'clock position.